1. Field of the Invention
The present invention relates to techniques for use in communication a system. More particularly, the present invention relates to techniques for identifying base stations in a communication system.
2. Description of the Related Art
In a typical wireless communication system, a service, such as a voice and/or data service, is provided to Mobile Stations (MSs) through a plurality of Macrocell Base Stations (MBSs). An MS may also be referred to as an Advanced Mobile Station (AMS) and an MBS may also be referred to as a Macrocell Advanced Base Station (MABS). Each of the MBSs is responsible for providing a service to MSs located within their respective service coverage area. The service coverage area of an MBS is hereafter referred to as a Macrocell. To facilitate mobility of the MSs, handoff between MBSs is performed when the MS leaves one Macrocell for another.
In the wireless communication system, a channel may deteriorate due to a number of factors, including a geographical factor inside a Macrocell, a distance between an MS and an MBS, movement of the MS, etc. The channel deterioration is problematic since it may result in a disruption of communication between the MS and the MBS. For example, when the MS is located inside a structure, such as an office building or a house, a channel between the MBS and the MS may deteriorate due a shadow region that is formed by the structure. A shadow region formed within the structure is hereafter referred to as an indoor shadow region. The MS located in the indoor shadow region may not be able to adequately perform communication with the MBS. Further, an MBS may have inadequate capacity to service all users in its Macrocell. To address the shortcomings of the MBS, a variety of other Base Stations (BSs) have been proposed, including a Relay, Picocell, Microcell, Femtocell, Ubicell etc. The Femtocell concept, as an example of a BS other than an MBS, will be explained further below.
The Femtocell concept is proposed to provide ubiquitous connectivity to MSs and improve wireless capacity, while addressing a service limitation of an indoor shadow region. A Femtocell is a small cell coverage area that is serviced by a low power Femtocell BS (FBS) that accesses a wireless communication Core Network (CN) via a commercial broadband network or via a wireless connection to a backhaul of the wireless communication system. Hereafter, the term “Femtocell” may be used interchangeably with the term “FBS.” Also, an FBS may be referred to as a Femtocell Advanced Base Station (FABS). The deployment of the Femtocell improves both the coverage and capacity of the wireless communication system. Going forward, the advantages of the Femtocell are expected to be increasingly leveraged in wireless communication systems. Since the Femtocell is much smaller than a Macrocell, a plurality of Femtocells may coexist within one Macrocell. The FBS is capable of providing service to relatively a small number of MSs, unlike the MBS, which is capable of providing service to a large number of MSs. The FBS typically operates in a licensed spectrum and may use the same or different frequency as the MBS. Further, MSs serviced by an FBS are typically stationary or moving at low (i.e., pedestrian) speed. Similar to the need for handoff between MBSs, handoff between the Femtocell and the Macrocell, and handoff between Femtocells, is a key function in securing service continuity of an MS.
The FBS may be installed inside or adjacent to an indoor space to which it is intended to provide service, such as a home or Small Office/Home Office (SOHO). Installation of the FBS is significantly easier than installation of the MBS and the FBS may be purchased and installed by a subscriber for use in conjunction with the wireless communication system. Here, the subscriber or service provider may desire to limit access to the FBS and only provide access to authorized MSs. To facilitate this arrangement, a Closed Subscriber Group (CSG) FBS may be employed. The CSG FBS can be further categorized as a CSG-closed and a CSG-open FBS. A CSG-closed FBS may only provide access to authorized MSs, except for emergency services and National Security/Emergency Preparedness (NS/EP) services. A CSG-open FBS (which may be referred to as a hybrid FBS in some systems) may, in addition, allow non-subscribers of the CSG a low priority access or a best-effort access, to guarantee the Quality of Service (QoS) of the subscribers. Alternatively, an Open Subscriber Group (OSG) FBS may be employed that provides access to any MS.
The FBS has different operational requirements than the MBS, the specifics of which may differ depending on the wireless communication system the FBS is implemented in. For example, in a wireless communication system operating according to the Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard, it is required that the air interface support features needed to limit MSs scanning for FBSs. Further it is required that the air interface support MSs in being able to access and HandOver (HO) to CSG FBSs if they are designated as part of the CSG. In addition, it is required that the air interface support preferred access and HO of MSs to their designated FBSs. Also, it is required that the air interface support optimized and seamless session continuity and HO between FBSs and Wireless Fidelity (WiFi) access systems. Further, it is required that the air interface allows for a dense deployment of a large number of Femtocells in a wireless communication system.
Given the above requirements for the Femtocell, which differ from the requirements of the Macrocell, there are a number of issues that arise with the deployment of the Femtocell in a wireless communication system. For example, one issue that arises with the deployment of the Femtocell in a wireless communication system is the need to identify BSs. However, not only is there a need to distinguish FBSs from MBSs, but there is also a need to distinguish open-access FBSs (which any MS is allowed to access) from CSG FBSs (which allows only authorized MSs to access), since some operations (e.g., HO, paging, etc.) may be different for FBSs and MBSs, and for open-access and CSG FBSs. For example, an MS moving at a high speed may not need to HO to any FBS. In another example, open-access FBSs may accept HO requests from any MS while an MS that is not authorized to access a CSG FBS should not send a HO request to that CSG FBS.
It has been proposed to use different groups of preambles to distinguish the types of the cells. The preambles are divided into groups, each group for one type of cell.
It has also been proposed in IEEE 802.16 discussions to use one bit of the Least Significant Bit (LSB), which is a programmable 24 bit segment of the Base Station IDentifier (BS ID), to distinguish FBSs and MBSs, and to use another bit of the LSB to distinguish open-access and CSG FBSs. Here the BS ID is a globally unique ID in an IEEE 802.16 system, with a 24 bit Most significant Bit (MSB) as Operator ID and 24 bit LSB as the ID within the Operator ID.
It has further been proposed to use at least one of different scrambling sequences and different Cyclic Redundancy Check (CRCs) to differentiate FBSs from MBSs, and to differentiate open-access and CSG FBSs.
It has further been proposed that a short FBS ID may be used to conserve overhead of HandOver (HO) messages. However, the structure of such a short FBS ID as not yet been proposed. In addition, how to handle CSG IDs has not yet been proposed.
Despite the various proposals described above, the problem of how to enable an MS to efficiently (with minimum overhead in signaling) identify whether it is authorized to access a CSG FBS, has not been fully addressed. The ability for an MS to identify whether it is authorized to access a CSG FBS is important because it affects which system operations the MS will perform.
The CSG ID is an important ID of a CSG FBS, which can be a CSG-closed or a CSG-open (hybrid) FBS. The CSG ID identifies a closed subscription group. For example, if a business owns several CSG FBSs, there can be a CSG ID assigned to all these CSG FBSs owned by the business. The CSG FBS needs to send the CSG ID to the MS so that the MS may determine whether it is authorized to access the CSG FBS. The reason the CSG ID needs to be sent to the MS is that, even if an MS knows that the FBS is a CSG FBS, the MS still needs to determine whether it has subscribed the CSG, i.e., whether it is authorized to access this CSG FBS in CSG-closed FBS case, or whether it is authorized to have higher priority to access this CSG FBS in CSG-open FBS case. To do this, the MS may have configured and stored therewith a white list of the CSG FBSs which the MS has subscribed. Accordingly, when the MS receives the CSG ID of a CSG FBS, the MS compares the received CSG ID against its white list of CSG IDs of the CSG FBSs the MS has subscribed. If the received CSG ID is in the white list of CSG IDs of the CSG FBSs the MS has subscribed, the MS knows that it has subscribed this CSG FBS. Hence, a CSG ID of the CSG FBS should be sent to the MS. To keep the white list of CSG IDs of CSG FBSs which the MS has subscribed short, multiple CSG FBSs may share a common CSG ID if they have the same set of subscriber MSs.
It has been proposed that the CSG ID, as an additional ID which is independent of the globally unique BS ID, that the FBS should broadcast, is broadcasted over a Broadcast CHannel (BCH), as the payload of the BCH. This introduces overhead on BCH.
It has been proposed to use the CSG IDs as different scrambling sequences to scramble a CRC of a BCH, so that the MS may differentiate CSG FBSs and determine whether it is authorized to access a given CSG FBS.
Another possible way for the MS to determine whether it is authorized to access a given CSG FBS is for the MS to have a white list of BS IDs of CSG FBSs the MS is authorized to access. Accordingly, when the MS receives the BS ID of a CSG FBS, the MS compares the received BS ID against its white list of BS IDs of the CSG FBSs the MS has subscribed. If the received CSG FBS ID is in the white list of BS IDs of the CSG FBSs the MS is authorized to access, the MS knows that it is authorized to access this CSG FBS. Note that a globally unique BS ID may be needed for an MS to access an FBS securely and for some other purposes. If the MS stores globally unique BS IDs, the white list may end up being very long. For example, consider the situation where an FBS is located at each Starbucks™ location, of which there may be hundreds or thousands of locations throughout the world. In this case, the white list will end up being very long.
Accordingly, using the CSG IDs over the BS IDs to determine if an MS is authorized to access a CSG FBS, could shorten the white list at the MS. In addition, use of the CSG ID to determine if an MS is authorized to access a CSG FBS may provide for easier management. Consider the example given above of an FBS located at each Starbucks™ location, when a CSG ID is not used to determine if an MS is authorized to access a CSG FBS. Here, when Starbucks™ installs a new CSG FBS, Starbucks™ has to request that the operator update the white list of all of its membership subscribers to add this new CSG FBS. However, if a CSG ID is used to determine if an MS is authorized to access a CSG FBS, such update is not needed.
Nevertheless, if the CSG ID is used for the MS to determine if the MS is authorized to access a CSG FBS, the CSG ID may be of a length similar to the BS ID. This may occur because, in the worst case scenario, one CSG ID will be needed for each FBS, such as in a home use case. In addition, since the CSG ID may not offer a globally unique BS ID for a CSG FBS for e.g., membership use cases, the globally unique BS ID is still needed for uniquely identifying FBSs in order to have secure access to an FBS and for other purposes. Accordingly, the worst case is that both the CSG ID and the globally unique BS ID, which can be of similar length, need to be broadcast.
Herein, there is need to efficiently provide both the CSG ID of the CSG FBS and the globally unique BS ID to the MS. However, such improvement should take diverse Femtocell use cases into consideration. Examples of such use cases include a home use case, an enterprise use case, a membership use case, an operation use case, etc. An example of the home use case is where a home owner purchases an FBS and authorizes access to only the MSs of family members and friends. An example of the enterprise use case is where a company purchases tens or hundreds of FBSs to enhance connectivity and authorizes access to only the MSs of employees of the company. An example of the membership use case is where a user may purchase a membership to access FBSs at Starbucks™, which means that the user should be authorized to access all FBSs owned by Starbucks™. An example of the operator use case is where the operator uses FBSs to fix service coverage holes. In this case, all subscribers of this operator should be authorized to access those FBSs.
Given these diverse use cases, if the CSG ID is of a fixed length, then the length of the CSG ID would be similar to the length of BS ID because in the worst case scenario, such as the home use case, the globally unique BS ID is still needed for e.g., the membership use case, to uniquely identify the CSG FBS.
Therefore, a need exists for a technique to efficiently provide the MS with both the CSG ID of the CSG FBS and the globally unique BS ID over the air, for diverse Femtocell use cases.